It is known that the emission frequency of a semiconductor laser undergoes fluctuations, dependent in particular on temperature variations in the active region. In a number of applications, e.g. coherent communications on optical fiber, such fluctuations must be controlled within rather narrow limits, and hence such lasers are associated with devices for automatic frequency control.
The most widely used control devices uses Fabry-Perot interferometers as frequency discriminators, associated with electronic circuits generating error signals used to control laser injection current or temperature. An example is described in the paper entitled "Frequency stability measurements of feedback stabilized AlGaAs DH lasers" by H. Tsuchida, S. Sanpei, M. Ohtsu, T. Tako, Japanese Journal of Applied Physics, Vol. 19, No. 12, Dec. 1980, pages L721-L724. In this device, a photomultiplier collects the beam emitted from the laser after such beam has been made to pass through a high stability Fabry-Perot interferometer, whose length is piezoelectrically modulated by a sinusoidal signal; a lock-in amplifier generates the first derivative of the interferometer spectrum by detecting the photomultiplier output signal synchronously with the modulating oscillation. Such a derivative becomes zero when the emission frequency is the nominal one, and is different from zero in the other cases. Thus an error signal is obtained, which consists in of the instantaneous value of the derivative in its linear portion, and such a signal is used to control the laser temperature.
This known system has a number of drawbacks. The scanning carried out by the interferometer is very short (and actually it corresponds to the portion of the photodetector output peak comprised between the two inflexion points). As a consequence there is a very small range in which frequency locking occurs and hence the system has a limited correction capability. Besides, if the laser linewidth or emitting power changes, the slope of the involved curve portion changes too; that affects the control system performance. Owing to the interferometer length modulation technique, the signal derivative can be obtained only by the use of expensive means, such as a lock-in amplifier. Lastly, this known system cannot be used to control a plurality of sources, as can be required in a multichannel transmission system. In effect, the technique adopted for generating the error signal, according to which a small frequency range about the maximum of the photomultiplier output peak is examined does not allow the analysis of a sequence of peaks having the frequency spacing required by such systems.
As to the latter problem, the article entitled "Optical FDM transmission technique" by K. Nosu, H. Toba and K. Ivashita, Journal of Lightwave Technology", Vol. Lt-5, No. 9, Sept. 1987, pages 1301-1308, discloses a device for the automatic frequency control of a plurality of lasers, used as sources in a frequency division multiplex optical communications system. In this device the heats are generated between the output signal of one of the lasers, stabilized in a conventional way and used as reference, and the signals emitted by the other lasers; the central heat frequencies are determined by a Fabry-Perot interferometer, and the information thus obtained is converted into digital electrical signals from which, after conversion into analog form, control signals for the other lasers are obtained. The known device is very complicated since it requires two stabilizing systems, one for the laser used as reference and the other for the remaining lasers; besides, since stabilization of said remaining lasers depends on that of the reference laser, the system performance depends on the reference stabilization precision.